In the process of electrophotographic printing, a charge-retentive surface, also known as a photoreceptor, is charged to a substantially uniform potential, so as to sensitize the surface of the photoreceptor. The charged portion of the photoconductive surface is exposed to a light image of an original document being reproduced, or else a scanned laser image created by the action of digital image data acting on a laser source. The scanning or exposing step records an electrostatic latent image on the photoreceptor corresponding to the informational areas in the document to be printed or copied. After the latent image is recorded on the photoreceptor, the latent image is developed by causing toner particles to adhere electrostatically to the charged areas forming the latent image. This developed image on the photoreceptor is subsequently transferred to a sheet on which the desired image is to be printed. Finally, the toner on the sheet is heated to permanently fuse the toner image to the sheet.
One familiar type of development of an electrostatic image is called “two-component development.” Two-component developer material largely comprises toner particles interspersed with carrier particles. The carrier particles may be attracted magnetically and the toner particles adhere to the carrier particles through triboelectric forces. This two-component developer can be conveyed, by means such as a “magnetic roll,” to the electrostatic latent image, where toner particles become detached from the carrier particles and adhere to the electrostatic latent image.
In magnetic roll development systems, the carrier particles with the triboelectrically adhered toner particles are transported by the magnetic rolls through a development zone. The development zone is the area between the outside surface of a magnetic roll and the photoreceptor surface on which a latent image has been formed. Because the carrier particles are attracted to the magnetic roll, some of the toner particles are interposed between a carrier particle and the latent image on the photoreceptor. These toner particles are attracted to the latent image and transfer from the carrier particles to the latent image. The carrier particles are removed from the development zone as they continue to follow the rotating surface of the magnetic roll. The carrier particles then fall from the magnetic roll and return to the developer supply where they attract more toner particles and are reused in the development process. The carrier particles fall from the magnetic roll under the effects of gravity or are directed away from the roller surface by a magnetic field.
One type of carrier particle used in two-component developers is the semi-conductive carrier particle. Developers using this type of carrier particle are also capable of being used in magnetic roll systems that produce toner bearing substrates at speeds of up to approximately 200 pages per minute (ppm). Developers having semi-conductive carrier particles use a relatively thin layer of developer on the magnetic roll in the development zone. In these systems an AC electric waveform is applied to the magnetic roller to cause the developer to become electrically conductive during the development process. The electrically conductive developer increases the efficiency of development by preventing development field collapse due to countercharge left in the magnetic brush by the developed toner. A typical waveform applied to these systems is, for example, a square wave at a peak to peak amplitude of 1000 Volts and a frequency of 9 KHz. This waveform controls both the toner movement and the electric fields in the development zone. These systems may be run in a “with” mode, which means the magnetic roll surface runs in the same direction as the photoreceptor surface, or in an “against” mode, which means the magnetic roll surface runs in a direction that is the opposite direction in which the photoreceptor surface runs. The high surface speed at which these magnetic rolls are operated require high strength magnets to control the developer bed. These types of magnets are expensive. Additionally, high speeds also increase the wear on bearings in the developer housing.
Another issue in known magnetic roll systems used with developers having semi-conductive carrier particles is the difficulty in extending the development zone to increase the time in which toner development may occur. One method for increasing development zone length with other developers having insulated or conductive carrier particles is to use two magnetic rolls. The two rolls are placed close together with their centers aligned to form a line that is parallel to the photoreceptor. Because the developer layer for semi-conductive carrier particle developer is so thin, magnetic fields sufficiently strong enough to cause semi-conductive carrier particles to migrate in adequate quantities from one magnetic roll to the other magnetic roll also interfere with the transfer of toner from the carrier particles in the development zones. Consequently, construction of the magnetic rolls requires careful consideration of this interference. If two rolls are not able to be used to increase the development zone, then the radius of the magnetic roll may be increased to accommodate this goal. There is a limit, however, to the diameter of the magnetic roll. One limit is simply the area within the printing machine that is available for a development subsystem. Another limit is the size and strength of the magnets internal to the magnetic roll that are required to provide adequate magnetic field strengths and shapes at the surface of a larger magnetic roll.
To address the issues arising in development systems having two magnetic development rolls, a development station has been implemented that increases the time for developing the toner and provides an adequate supply of developer for good line detail, edges, and solids. The development system includes an upper magnetic developer roller and a lower magnetic developer roller. Both developer rollers have a stationary core with at least one magnet and a sleeve that rotates about the stationary core. A motor coupled to the two magnetic developer rolls drives the rotating sleeves of the magnetic developer rolls in a direction that is against the rotational direction of a photoreceptor to which the two magnetic rolls deliver toner. The two magnetic developer rolls carry semi-conductive carrier particles and toner particles through a development zone formed by the magnetic developer rolls. A trim blade is mounted proximate the upper magnetic developer roll to form a trim gap of approximately 0.5 to approximately 0.75 mm.
This development station architecture has resulted in improved development for electrostatographic imaging machines. Unfortunately, a weakness has been discovered in the two vertical roll architecture described above. The observed weakness relates to the loss of carrier particles as the developer transitions from the upper roll to the lower roll. Specifically, the carrier particles are driven out of the system at the ends of the zone between the two rolls. These escaping particles contaminate the internal components of the imaging machine, particularly, the backer bars, the belt to roll spacer (BRS), and the front panel of the machine.
Previously known development stations have used magnetic seals that are mounted at the ends of a development roller. These magnetic seals are comprised of magnetic materials that generate fields for impeding the outward progression of carrier particles at the ends of the roller. In the two roller vertical architecture, these magnetic seals do not generate adequate fields in the vicinity of the space between the two rollers. Space constraints between the rollers inhibit the extension of the magnetic seals at the ends of the rollers. Consequently, carrier particles escape at the ends of the gap between the two rollers with the attendant issues previously noted.
The system and method discussed below address the loss of carrier particles from development stations having two vertically arranged magnetic developer rolls.